An Electrically Erasable Programmable Read Only Memory (EEPROM) is a type of nonvolatile memory that is reprogrammable. EEPROM cells are arranged as an array and located at the intersections of rows and columns. An EEPROM cell generally comprises a floating gate transistor and a select transistor. Programming and erasing of the EEPROM cell is accomplished by applying a high voltage to the control electrode of the floating gate transistor. During programming, the select transistor is used to isolate the unselected EEPROM cells that are located on the same column. A programming voltage as high as 18-20 volts may be needed to perform a program or erase operation.
A detailed description of a prior art charge pump is described in the specification and in FIGS. 1 and 2 of a related, copending application entitled "High Voltage Charge Pump", invented by Bruce Lee Morton, having Ser. No. 08/028,000, filed Mar. 8, 1993, and assigned to the assignee hereof, which is expressly incorporated by this reference. Some prior art charge pumps became less efficient for power supply voltages less than about 3.75 volts, and in fact do not function adequately for low power supply voltages, such as 3.3 volts.
In fact, most prior art charge pumps either do not function properly or cannot generate a sufficiently high voltage output (18 volts or more) below a power supply voltage of approximately 2.0 volts. There are two main problems with typical prior art charge pumps which use a series of stages to build up the voltage output. First, as the power supply voltage drops, charge transfer between the stages becomes less efficient. This reduction in efficiency is due to the fact that the clock inputs to circuits which control the charge transfer are weakened as the power supply voltage drops. Second, amount of charge which can be stored on capacitors in the stages is decreased as the power supply voltage drops. Therefore, these drawbacks of prior art charge pumps cause the final voltage output of the prior art charge pump to be reduced. If the final voltage output of the prior art charge pump is reduced too much, the prior art charge pump may not be able to successfully program or erase the EEPROM cells.